For water treatment, there exist several typical kinds of pretreatment apparatuses and pretreatment methods. As an example, for biological wastewater treatment apparatuses, there are pretreatment apparatuses such as apparatuses performing settling, filtration, pH control, ozone oxidization and adsorption.
The object of these pretreatment apparatuses is to reduce biological, chemical, or physical load on the wastewater treatment apparatus in the next step. The expected effects of these apparatuses include downsizing of the wastewater treatment apparatuses, reduction in running costs, and improvement of the water quality of the treated water from the wastewater treatment apparatuses.
However, the conventional pretreatment apparatuses cannot achieve considerable increase of the micro-nano bubble concentration in water under treatment so as to sustain high micro-nano bubble concentration over a long time till the next step, nor perform treatment with new functions such as a cleaning function and a weak sterilization function for membrane devices. Herein, the micro-nano bubbles includes both the micro bubbles with a diameter equal to or smaller than 50 microns and larger than 1 micron and the nano bubbles with a diameter equal to or smaller than 1 micron.
Moreover, the conventional pretreatment apparatuses have a general aeration function by a blower but do not have a treatment function with the micro-nano bubbles. It is to be noted that the pretreatment apparatuses with the micro-nano bubbles have a function to maintain the concentration of dissolved oxygen high till the next step for a long time.
Conventional method and apparatus for using nano bubbles have been disclosed in JP 2004-121962 A. This conventional method and apparatus for using nano bubbles utilize such characteristics of nano bubbles as decrease in buoyancy, increase in surface area, increase in surface activity, generation of local high pressure fields, a surface active property attained by achievement of electrostatic polarization and an antiseptic property. More specifically, it has been disclosed that by associating these characteristics with each other, a fouling component adsorption function, a substance surface high-speed cleaning and an antiseptic function allow advanced cleaning of various substances with low environmental load so that purification of contaminated water becomes possible.
However, it has not been disclosed:                (1) that the micro-nano bubbles are generated in a micro-nano bubble generation tank, and the water under treatment containing the micro-nano bubbles is introduced into an anaerobic measuring tank so as to optimize the generation state of the micro-nano bubbles with use of values of dissolved oxygen concentration and oxidation reduction potential; or        (2) that in stages prior to each of (a) an ultrapure water production apparatus composed of a pretreatment apparatus, a primary pure water production apparatus and a secondary water purifying apparatus, (b) a dilute wastewater recovering apparatus, (c) a general service water recovering apparatus, and (d) a wastewater treatment apparatus, a treatment tank which is composed of a micro-nano bubble generation tank and an anaerobic measuring tank is placed.        
Further, there is a method for generating nano air bubbles disclosed in JP 2003-334548 A. This method for generating nano air bubbles includes step (i) for gasifying part of liquid by decomposition in liquids, step (ii) for applying ultrasonic waves in liquids, or step (iii) composed of a step for gasifying part of liquid by decomposition and a step for applying ultrasonic waves.
However, it has not been disclosed that:                (3) that the micro-nano bubbles are generated in the micro-nano bubble generation tank, and the water under treatment containing the micro-nano bubbles is introduced into an anaerobic measuring tank to optimize the generation state of the micro-nano bubbles with use of values of dissolved oxygen concentration and oxidation reduction potential; or        (4) that in stages prior to each of (a) an ultrapure water production apparatus composed of a pretreatment apparatus, a primary pure water production apparatus and a secondary water purifying apparatus, (b) a dilute wastewater recovering apparatus, (c) a general service water recovering apparatus, and (d) a wastewater treatment apparatus, a treatment tank which is composed of a micro-nano bubble generation tank and an anaerobic measuring tank is placed.        
Further, there is an apparatus for treating waste fluid disclosed in JP 2004-321959 A. In this apparatus for treating waste fluid, ozone gas generated by an ozonizer and waste liquid drawn from the bottom of a treatment tank are fed to a micro bubble generator through a pressurization pump. It has also been disclosed that the waste fluid in the treatment tank is aerated by the generated ozone micro bubbles sent from the opening of a gas blow-off pipe.
However, it has not been disclosed:                (5) that the micro-nano bubbles are generated in the micro-nano bubble generation tank, and the water under treatment containing the micro-nano bubbles is introduced into an anaerobic measuring tank to optimize the generation state of the micro-nano bubbles with use of values of dissolved oxygen concentration and oxidation reduction potential; or        (6) that in stages prior to each of (a) an ultrapure water production apparatus composed of a pretreatment apparatus, a primary pure water production apparatus and a secondary water purifying apparatus, (b) a dilute wastewater recovering apparatus, (c) a general service water recovering apparatus, and (d) a wastewater treatment apparatus, a treatment tank which is composed of a micro-nano bubble generation tank and an anaerobic measuring tank is placed.        
Although various types of pretreatment apparatuses are conventionally present as the pretreatment apparatus for membrane devices as described above, no pretreatment apparatus is present which is capable of effectively preventing clogging phenomenon of membrane devices and enhancing the capability of the membrane devices with use of simple devices which are low in cost, easy to maintain and energy saving.